Features of contralaterally evoked inhibition in the inferior colliculus

Spatial Separation between Two Sounds of an Oddball Paradigm Affects Responses of Neurons in the Rat's Inferior Colliculus to the Sounds

The ability to sense occasionally occurring sounds in an environment is critical for animals. To understand this ability, we studied responses to acoustic oddball paradigms in the rat's midbrain auditory neurons. An oddball paradigm is a random sequence of stimuli created using two tone bursts, with one presented at a high probability (standard stimulus) and the other at a low probability (oddball stimulus). The sounds were either colocalized at the ear contralateral to a neuron under investigation (c90° azimuth) or separated with one at c90° while the other at another azimuth. We found that most neurons generated stronger responses to a sound at c90° when it was presented as an oddball than as a standard stimulus. Relocating one sound from c90° to another azimuth changed both responses to the relocated sound and the sound that remained at c90°. Most notably, the response to an oddball stimulus at c90° was increased when a standard stimulus was relocated from c90° to a location that was in front of the animal or on the ipsilateral side of recording. The increase was particularly large in neurons that displayed transient firing under contralateral stimulation but no firing under ipsilateral stimulation. These neurons likely play a particularly important role in using spatial cues to detect occasionally occurring sounds. Results suggest that effects of spatial separation between two sounds of an oddball paradigm on responses to the sounds were dependent on changes in the level of adaptation and binaural inhibition.

Processing of Natural Echolocation Sequences in the Inferior Colliculus of Seba's Fruit Eating Bat, Carollia perspicillata

For the purpose of orientation, echolocating bats emit highly repetitive and spatially directed sonar calls. Echoes arising from call reflections are used to create an acoustic image of the environment. The inferior colliculus (IC) represents an important auditory stage for initial processing of echolocation signals. The present study addresses the following questions: (1) how does the temporal context of an echolocation sequence mimicking an approach flight of an animal affect neuronal processing of distance information to echo delays? (2) how does the IC process complex echolocation sequences containing echo information from multiple objects (multiobject sequence)? Here, we conducted neurophysiological recordings from the IC of ketamine-anaesthetized bats of the species Carollia perspicillata and compared the results from the IC with the ones from the auditory cortex (AC). Neuronal responses to an echolocation sequence was suppressed when compared to the responses to temporally isolated and randomized segments of the sequence. The neuronal suppression was weaker in the IC than in the AC. In contrast to the cortex, the time course of the acoustic events is reflected by IC activity. In the IC, suppression sharpens the neuronal tuning to specific call-echo elements and increases the signal-to-noise ratio in the units' responses. When presenting multiple-object sequences, despite collicular suppression, the neurons responded to each object-specific echo. The latter allows parallel processing of multiple echolocation streams at the IC level. Altogether, our data suggests that temporally-precise neuronal responses in the IC could allow fast and parallel processing of multiple acoustic streams.

Responses from two firing patterns in inferior colliculus neurons to stimulation of the lateral lemniscus dorsal nucleus

The γ-aminobutyric acid neurons (GABAergic neurons) in the inferior colliculus are classified into various patterns based on their intrinsic electrical properties to a constant current injection. Although this classification is associated with physiological function, the exact role for neurons with various firing patterns in acoustic processing remains poorly understood. In the present study, we analyzed characteristics of inferior colliculus neurons in vitro, and recorded responses to stimulation of the dorsal nucleus of the lateral lemniscus using the whole-cell patch clamp technique. Seven inferior colliculus neurons were tested and were classified into two firing patterns: sustained-regular (n = 4) and sustained-adapting firing patterns (n = 3). The majority of inferior colliculus neurons exhibited slight changes in response to stimulation and bicuculline. The responses of one neuron with a sustained-adapting firing pattern were suppressed after stimulation, but recovered to normal levels following application of the γ-aminobutyric acid receptor antagonist. One neuron with a sustained-regular pattern showed suppressed stimulation responses, which were not affected by bicuculline. Results suggest that GABAergic neurons in the inferior colliculus exhibit sustained-regular or sustained-adapting firing patterns. Additionally, GABAergic projections from the dorsal nucleus of the lateral lemniscus to the inferior colliculus are associated with sound localization. The different neuronal responses of various firing patterns suggest a role in sound localization. A better understanding of these mechanisms and functions will provide better clinical treatment paradigms for hearing deficiencies.

The superior paraolivary nucleus shapes temporal response properties of neurons in the inferior colliculus

The mammalian superior paraolivary nucleus (SPON) is a major source of GABAergic inhibition to neurons in the inferior colliculus (IC), a well-studied midbrain nucleus that is the site of convergence and integration for the majority ascending auditory pathways en route to the cortex. Neurons in the SPON and IC exhibit highly precise responses to temporal sound features, which are important perceptual cues for naturally occurring sounds. To determine how inhibitory input from the SPON contributes to the encoding of temporal information in the IC, a reversible inactivation procedure was conducted to silence SPON neurons, while recording responses to amplitude-modulated tones and silent gaps between tones in the IC. The results show that SPON-derived inhibition shapes responses of onset and sustained units in the IC via different mechanisms. Onset neurons appear to be driven primarily by excitatory inputs and their responses are shaped indirectly by SPON-derived inhibition, whereas sustained neurons are heavily influenced directly by transient offset inhibition from the SPON. The findings also demonstrate that a more complete dissection of temporal processing pathways is critical for understanding how biologically important sounds are encoded by the brain.

Circuit models and experimental noise measurements of micropipette amplifiers for extracellular neural recordings from live animals

Glass micropipettes are widely used to record neural activity from single neurons or clusters of neurons extracellularly in live animals. However, to date, there has been no comprehensive study of noise in extracellular recordings with glass micropipettes. The purpose of this work was to assess various noise sources that affect extracellular recordings and to create model systems in which novel micropipette neural amplifier designs can be tested. An equivalent circuit of the glass micropipette and the noise model of this circuit, which accurately describe the various noise sources involved in extracellular recordings, have been developed. Measurement schemes using dead brain tissue as well as extracellular recordings from neurons in the inferior colliculus, an auditory brain nucleus of an anesthetized gerbil, were used to characterize noise performance and amplification efficacy of the proposed micropipette neural amplifier. According to our model, the major noise sources which influence the signal to noise ratio are the intrinsic noise of the neural amplifier and the thermal noise from distributed pipette resistance. These two types of noise were calculated and measured and were shown to be the dominating sources of background noise for in vivo experiments.

Local inhibition of GABA affects precedence effect in the inferior colliculus

The precedence effect is a prerequisite for faithful sound localization in a complex auditory environment, and is a physiological phenomenon in which the auditory system selectively suppresses the directional information from echoes. Here we investigated how neurons in the inferior colliculus respond to the paired sounds that produce precedence-effect illusions, and whether their firing behavior can be modulated through inhibition with gamma-aminobutyric acid (GABA). We recorded extracellularly from 36 neurons in rat inferior colliculus under three conditions: no injection, injection with saline, and injection with gamma-aminobutyric acid. The paired sounds that produced precedence effects were two identical 4-ms noise bursts, which were delivered contralaterally or ipsilaterally to the recording site. The normalized neural responses were measured as a function of different inter-stimulus delays and half-maximal interstimulus delays were acquired. Neuronal responses to the lagging sounds were weak when the inter-stimulus delay was short, but increased gradually as the delay was lengthened. Saline injection produced no changes in neural responses, but after local gamma-aminobutyric acid application, responses to the lagging stimulus were suppressed. Application of gamma-aminobutyric acid affected the normalized response to lagging sounds, independently of whether they or the paired sounds were contralateral or ipsilateral to the recording site. These observations suggest that local inhibition by gamma-aminobutyric acid in the rat inferior colliculus shapes the neural responses to lagging sounds, and modulates the precedence effect.

Circuits that innervate excitatory-inhibitory cells in the inferior colliculus obtained with in vivo whole cell recordings

Neurons excited by stimulation of one ear and suppressed by the other, called excitatory/inhibitory (EI) neurons, are sensitive to interaural intensity disparities, the cues animals use to localize high frequencies. EI neurons are first formed in lateral superior olive, which then sends excitatory projections to the dorsal nucleus of the lateral lemniscus and the inferior colliculus (IC), both of which contain large populations of EI cells. We evaluate herein the inputs that innervate EI cells in the IC of Mexican free-tailed bats (Tadarida brasilensis mexicana) with in vivo whole-cell recordings from which we derived excitatory and inhibitory conductances. We show that the basic EI property in the majority of IC cells is inherited from lateral superior olive, but that each type of EI cell is also innervated by the ipsilateral or contralateral dorsal nucleus of the lateral lemniscus, as well as additional excitatory and inhibitory inputs from monaural nuclei. We identify three EI types, each of which receives a set of projections that is different from the other types. To evaluate the role that the various projections played in generating binaural responses, we used modeling to compute a predicted response from the conductances. We then omitted one of the conductances from the computation to evaluate the degree to which that input contributed to the binaural response. We show that the formation of the EI property in the various types is complex, and that some projections exert such subtle influences that they could not have been detected with extracellular recordings or even from intracellular recordings of postsynaptic potentials.

Selectivity and persistent firing responses to social vocalizations in the basolateral amygdala

This study examined responsiveness to acoustic stimuli among neurons of the basolateral amygdala. While recording from single neurons in awake mustached bats (Pteronotus parnellii), we presented a wide range of acoustic stimuli including tonal, noise, and vocal signals. While many neurons displayed phasic or sustained responses locked to effective auditory stimuli, the majority of neurons (n=58) displayed a persistent excitatory discharge that lasted well beyond stimulus duration and filled the interval between successive stimuli. Persistent firing usually began seconds (median value, 5.4 s) after the initiation of a train of repeated stimuli and lasted, in the majority of neurons, for at least 2 min after the end of the stimulus train. Auditory-responsive amygdalar neurons were generally excited by one stimulus or very few stimuli. Most neurons did not respond well to synthetic stimuli including tones, noise bursts or frequency-modulated sweeps, but instead responded only to vocal stimuli (82 of 87 neurons). Furthermore, most neurons were highly selective among vocal stimuli. On average, neurons responded to 1.7 of 15 different syllables or syllable sequences. The largest percentage of neurons responded to a hiss-like rectangular broadband noise burst (rBNB) call associated with aggressive interactions. Responsiveness to effective vocal stimuli was reduced or eliminated when the spectrotemporal features of the stimuli were altered in a subset of neurons. Chemical activation of the medial geniculate body (MG) increased both background and evoked firing. Among 39 histologically localized recording sites, we saw no evidence of topographic organization in terms of temporal response pattern, habituation, or the affect of calls to which neurons responded. Overall, these studies demonstrate that amygdalar neurons in the mustached bat show high selectivity to vocal stimuli, and suggest that persistent firing may be an important feature of amygdalar responses to social vocalizations.

It's about time: how input timing is used and not used to create emergent properties in the auditory system

The hypothesis for directional selectivity of frequency modulations (FMs) invokes a mechanism with an honored tradition in sensory neurobiology, the relative timing of excitation and inhibition. The proposal is that the timing disparity is created by asymmetrical locations of excitatory tuning and inhibitory sidebands. Thus, cells in which the inhibitory sidebands are tuned to frequencies lower than the excitatory tuning are selective for downward sweeping FMs, because frequencies first generate excitation followed by inhibition. Upward sweeping FMs, in contrast, first evoke inhibition that either leads or is coincident with the excitation and prevents discharges. Here we evaluated FM directional selectivity with in vivo whole-cell recordings from the inferior colliculus of awake bats. From the whole-cell recordings, we derived synaptic conductance waveforms evoked by downward and upward FMs. We then tested the effects of shifting inhibition relative to excitation in a model and found that latency shifts had only minor effects on EPSP amplitudes that were often <1.0 mV/ms shift. However, when the PSPs peaked close to spike threshold, even small changes in latency could cause some cells to fire more strongly to a particular FM direction and thus change its directional selectivity. Furthermore, the effect of shifting inhibition depended strongly on initial latency differences and the shapes of the conductance waveforms. We conclude that "timing" is more than latency differences between excitation and inhibition, and response selectivity depends on a complex interaction between the timing, the shapes, and magnitudes of the excitatory and inhibitory conductances and spike threshold.

Time dependence of binaural responses in the rat's central nucleus of the inferior colliculus

Recordings were made from single neurons in the rat's central nucleus of the inferior colliculus. Excitatory/inhibitory binaural interactions and interaural-level difference curves were determined for responses to 100 ms dichotic tone bursts presented to the left and right ears simultaneously. Most neurons with sustained responses to tone bursts had the same binaural response type throughout the 100 ms stimulus period. However, some neurons (39% of our sample) showed qualitatively different binaural response types during the early and late parts of the stimulus (the first 20 ms versus the last 80 ms of the tone burst). Also, for many neurons with consistent early and late binaural response patterns, the strength of binaural interaction was different during the early and late periods. For example, for neurons excited by the contralateral ear and inhibited by the ipsilateral ear during the entire 100 ms period (the most common binaural response type), the degree of inhibition was generally greater during the later part of a stimulus. This change in the strength and/or quality of binaural interaction during dichotic stimulation likely reflects a complex pattern of converging excitatory and inhibitory inputs to the inferior colliculus from lower brainstem structures as well as the time course of local synaptic events. The temporal properties of binaural interaction may influence how sound source location is represented in the central auditory system.

Neural correlates of context-dependent perceptual enhancement in the inferior colliculus

In certain situations, preceding auditory stimulation can actually result in heightened sensitivity to subsequent sounds. Many of these phenomena appear to be generated in the brain as reflections of central computations. One example is the robust perceptual enhancement (or "pop out") of a probe signal within a broadband sound whose onset time is delayed relative to the remainder of a mixture of tones. Here we show that the neural representation of such stimuli undergoes a dramatic transformation as the pathway is ascended, from an implicit and distributed peripheral code to explicitly facilitated single-neuron responses at the level of the inferior colliculus (IC) of two awake and passively listening female marmoset monkeys (Callithrix jacchus). Many key features of the IC responses directly parallel psychophysical measures of enhancement, including the dependence on the width of a spectral notch surrounding the probe, the overall level of the complex, and the duration of the preceding sound (referred to as the conditioner). Neural detection thresholds for the probe with and without the conditioner were also in qualitative agreement with analogous psychoacoustic measures. Response characteristics during the conditioners were predictive of the enhancement or suppression of the ensuing probe response: buildup responses were associated with enhancement, whereas adapting conditioner responses were more likely to result in suppression. These data can be primarily explained by a phenomenological computational model using dynamic (adapting) inhibition as a necessary ingredient in the generation of neural enhancement.

Time-dependent effects of ipsilateral stimulation on contralaterally elicited responses in the rat's central nucleus of the inferior colliculus

Recordings were made from single neurons in the rat's central nucleus of the inferior colliculus (ICc). Binaural responses were studied when dichotic tone bursts with various interaural-level differences were presented simultaneously or with a contralateral delay. These dichotic tone bursts allowed us to probe temporal changes in the effect produced by an ipsilateral sound on a contralaterally elicited response. Most of the neurons in the rat's ICc were excited by contralateral and inhibited by ipsilateral stimulation. For the majority of neurons with excitatory/inhibitory interactions, the early part of an ipsilateral stimulus caused stronger inhibition than the late part. The ipsilateral stimulus frequently produced an excitatory or inhibitory "offset" effect that was apparent soon after cessation of the stimulus. For many neurons, this aftereffect substantially changed the strength and temporal firing pattern of the response elicited by a lagging contralateral stimulus. Our results suggest that there are time-dependent changes in the effect of ipsilateral stimulation on the pattern and strength of responses to contralateral stimulation. These effects frequently outlast the duration of a leading ipsilateral stimulus. These characteristics of binaural interaction likely reflect the time courses of converging excitatory and inhibitory synaptic inputs to ICc neurons as well as the intrinsic membrane properties of those neurons.

Wide-dynamic-range forward suppression in marmoset inferior colliculus neurons is generated centrally and accounts for perceptual masking

An organism's ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input's response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

Rethinking tuning: in vivo whole-cell recordings of the inferior colliculus in awake bats

Tuning curves were recorded with patch electrodes from the inferior colliculus (IC) of awake bats to evaluate the tuning of the inputs to IC neurons, reflected in their synaptic tuning, compared with the tuning of their outputs, expressed in their discharge tuning. A number of unexpected features were revealed with whole-cell recordings. Among these was that most neurons responded to tones with inhibition and/or subthreshold excitation over a surprisingly broad frequency range. The synaptic tuning in many cells was at least 1.5-2.0 octaves wide and, on average, was more than twice as wide as the frequency range that evoked discharges even after inhibition was blocked. In most cells, tones evoked complex synaptic response configurations that varied with frequency, suggesting that these cells were not innervated by congruent excitatory and inhibitory projections. Synaptic tuning was not only wide but was also diverse, in which some cells were dominated by excitation (n = 20), others were dominated by excitation with sideband inhibition (n = 21), but most were dominated by inhibition with little evidence of excitation (n = 31). Another unexpected finding was that some cells responded with inhibition to the onset and offset of tones over a wide frequency range, in which the patterns of synaptic responses changed markedly with frequency. These cells never fired to tones at 50 dB sound pressure level but fired to frequency-modulated sweeps at that intensity and were directionally selective. Thus, the features revealed by whole-cell recordings show that the processing in many IC cells results from inputs spectrally broader and more complex than previously believed.

Is duration tuning a transient process in the inferior colliculus of guinea pigs?

Duration selectivity appears to be a fundamental neural encoding mechanism found throughout the animal kingdom. Previous studies reported that band-pass duration-tuned neurons typically show offset responses and occupy a small portion of auditory neurons in non-echolocation mammals relative to echolocation bats. Therefore, duration tuning is generally weaker in non-echolocation mammals. In the present study, duration tuning was analyzed for 207 neurons recorded in the inferior colliculus (IC) of guinea pigs. Duration tuning was found to be stronger in the onset component of the responses from sustained, on-off and pause neurons than had been reported previously, when a short analysis window was applied. The need for an appropriate time window for duration tuning analysis was also supported by the fact that the on and off responses from an on-off neuron may show different duration tuning features. Therefore, duration tuning appears to be a transient neural coding process in the IC of guinea pigs. Duration tuning for these types of neurons may have been blurred by the use of a relatively unselective, long window.

Serotonin shifts first-spike latencies of inferior colliculus neurons

Many studies of neuromodulators have focused on changes in the magnitudes of neural responses, but fewer studies have examined neuromodulator effects on response latency. Across sensory systems, response latency is important for encoding not only the temporal structure but also the identity of stimuli. In the auditory system, latency is a fundamental response property that varies with many features of sound, including intensity, frequency, and duration. To determine the extent of neuromodulatory regulation of latency within the inferior colliculus (IC), a midbrain auditory nexus, the effects of iontophoretically applied serotonin on first-spike latencies were characterized in the IC of the Mexican free-tailed bat. Serotonin significantly altered the first-spike latencies in response to tones in 24% of IC neurons, usually increasing, but sometimes decreasing, latency. Serotonin-evoked changes in latency and spike count were not always correlated but sometimes occurred independently within individual neurons. Furthermore, in some neurons, the size of serotonin-evoked latency shifts depended on the frequency or intensity of the stimulus, as reported previously for serotonin-evoked changes in spike count. These results support the general conclusion that changes in latency are an important part of the neuromodulatory repertoire of serotonin within the auditory system and show that serotonin can change latency either in conjunction with broad changes in other aspects of neuronal excitability or in highly specific ways.

Neurons in the medial nucleus of the trapezoid body and superior paraolivary nucleus of the rat may play a role in sound duration coding

We describe neurons in two nuclei of the superior olivary complex that display differential sensitivities to sound duration. Single units in the medial nucleus of the trapezoid body (MNTB) and superior paraolivary nucleus (SPON) of anesthetized rats were studied. MNTB neurons produced primary-like responses to pure tones and displayed a period of suppressed spontaneous activity after stimulus offset. In contrast, neurons of the SPON, which receive a strong glycinergic input from MNTB, showed very little or no spontaneous activity and responded with short bursts of action potentials after the stimulus offset. Because SPON spikes were restricted to the same time window during which suppressed spontaneous activity occurs in the MNTB, we presume that SPON offset activity represents a form of postinhibitory rebound. Using characteristic frequency tones of 2- to 1,000-ms duration presented 20 dB above threshold, we show that the profundity and duration of the suppression of spontaneous activity in MNTB as well as the magnitude and first spike latency of the SPON offset response depend on stimulus duration as well as on stimulus intensity, showing a tradeoff between intensity and duration. Pairwise comparisons of the responses to stimuli of various durations revealed that the duration sensitivity in both nuclei is sharpest for stimuli <50 ms.

Differing roles of inhibition in hierarchical processing of species-specific calls in auditory brainstem nuclei

Here we report on response properties and the roles of inhibition in three brain stem nuclei of Mexican-free tailed bats: the inferior colliculus (IC), the dorsal nucleus of the lateral lemniscus (DNLL) and the intermediate nucleus of the lateral lemniscus (INLL). In each nucleus, we documented the response properties evoked by both tonal and species-specific signals and evaluated the same features when inhibition was blocked. There are three main findings. First, DNLL cells have little or no surround inhibition and are unselective for communication calls, in that they responded to approximately 97% of the calls that were presented. Second, most INLL neurons are characterized by wide tuning curves and are unselective for species-specific calls. The third finding is that the IC population is strikingly different from the neuronal populations in the INLL and DNLL. Where DNLL and INLL neurons are unselective and respond to most or all of the calls in the suite we presented, most IC cells are selective for calls and, on average, responded to approximately 50% of the calls we presented. Additionally, the selectivity for calls in the majority of IC cells, as well as their tuning and other response properties, are strongly shaped by inhibitory innervation. Thus we show that inhibition plays only limited roles in the DNLL and INLL but dominates in the IC, where the various patterns of inhibition sculpt a wide variety of emergent response properties from the backdrop of more expansive and far less specific excitatory innervation.

Excitatory and inhibitory intensity tuning in auditory cortex: evidence for multiple inhibitory mechanisms

The intensity tuning of excitatory and suppressive domain frequency response areas was investigated in 230 cat primary auditory cortical and 92 posterior auditory field neurons. Suppressive domains were explored using simultaneous 2-tone stimulation with one tone at the best excitatory frequency. The intensity tuning of excitatory and suppressive domains was negatively correlated, supporting the hypothesis that inhibitory sidebands are related to excitatory domain intensity tuning. To further test this hypothesis, we compared the slopes of the edges of suppressive bands to the intensity tuning of excitatory domains. Edges of suppressive bands next to excitatory domains had slopes significantly more slanted toward the excitatory area in neurons with intensity-tuned excitatory domains. This relationship was not observed for suppressive band edges not next to the excitatory domain (e.g., the lower edge of lower suppressive bands). This indicates that intensity tuning ultimately observed in the excitatory domain results from overlapping excitatory and inhibitory inputs. In combination with results using forward masking, our results suggest that there are separate early and late sources of inhibition contributing to cortical frequency response areas, and only the early-stage inhibition contributes to excitatory domain intensity tuning.

Neuronal activity in the inferior colliculus and bordering structures during vocalization in the squirrel monkey

In four squirrel monkeys (Saimiri sciureus), the inferior colliculus, together with the neighboring superior colliculus, reticular formation, cuneiform nucleus and parabrachial area, were explored with microelectrodes, looking for neurons that might be involved in the discrimination between self-produced and external sounds. Vocalization was elicited by kainic acid injections into the periaqueductal gray of the midbrain. Acoustic tests were carried out with ascending and descending narrow-band noise sweeps spanning virtually the whole hearing range of the squirrel monkey. Altogether 577 neurons were analyzed. Neurons that both were audiosensitive and fired in advance of self-produced vocalization were found almost exclusively in the pericentral nuclei of the inferior colliculus and the adjacent reticular formation. Only the latter, however, contained, in addition, neurons that fired during external acoustic stimulation, but remained quiet during self-produced vocalization. These findings suggest that the reticular formation bordering the inferior colliculus is involved in the discrimination between self-produced and foreign vocalization on the basis of a vocalmotor feedforward mechanism.

Temporal masking reveals properties of sound-evoked inhibition in duration-tuned neurons of the inferior colliculus

The inferior colliculus (IC) is the first place in the central auditory pathway where duration-selective neurons are found. Previous neuropharmacological and electrophysiological studies have shown that they are created there and have led to a conceptual model in which excitatory and inhibitory inputs are offset in time so that the cell fires only when sound duration is such that onset- and offset-evoked excitation coincide; the response is suppressed by inhibition at other durations. We tested predictions from the model using paired tone stimulation and extracellular recording in the IC of the big brown bat, Eptesicus fuscus. Responses to a best duration (BD) tone were used as a probe to examine the strength and time course of inhibition activated by a nonexcitatory (NE) tone of the same frequency but differing in duration. As the relative time between the BD and NE tones was varied, the activity evoked by the BD tone was affected in ways comparable with backward, simultaneous, and forward masking. Responses to the BD tone were completely suppressed at short interstimulus intervals when the BD tone preceded the NE tone. Suppression was also seen when the stimuli temporally overlapped and summed and at intervals when the BD tone followed the NE tone. The results show that duration-selective neurons receive an onset-evoked, inhibitory input that precedes their excitatory input. The period of leading inhibition was correlated with BD and first spike latency. The results suggest how inhibition in the CNS could explain temporal masking phenomena, including backward masking.

Physiological response properties of neurons in the superior paraolivary nucleus of the rat

The superior paraolivary nucleus (SPON) is a prominent nucleus of the superior olivary complex. In rats, this nucleus is composed of a morphologically homogeneous population of GABAergic neurons that receive excitatory input from the contralateral cochlear nucleus and inhibitory input from the ipsilateral medial nucleus of the trapezoid body. SPON neurons provide a dense projection to the ipsilateral inferior colliculus and are thereby capable of exerting profound modulatory influence on collicular neurons. Despite recent interest in the structural and connectional features of SPON, little is presently known concerning the physiological response properties of this cell group or its functional role in auditory processing. We utilized extracellular, in vivo recording methods to study responses of SPON neurons to broad band noise, pure tone, and amplitude-modulated pure tone stimuli. Localization of recording sites within the SPON provides evidence for a medial (high frequency) to lateral (low frequency) tonotopic representation of frequencies within the nucleus. Best frequencies of SPON neurons spanned the audible range of the rat and receptive fields were narrow with V-shaped regions near threshold. Nearly all SPON neurons responded at the offset of broad band noise and pure tone stimuli. The vast majority of SPON neurons displayed very low rates of spontaneous activity and only responded to stimuli presented to the contralateral ear, although a small population showed binaural facilitation. Most SPON neurons also generated spike activity that was synchronized to sinusoidally amplitude-modulated tones. Taken together, these data suggest that SPON neurons may serve to encode temporal features of complex sounds, such as those contained in species-specific vocalizations.

Response selectivity for species-specific calls in the inferior colliculus of Mexican free-tailed bats is generated by inhibition

Here we show that inhibition shapes diverse responses to species-specific calls in the inferior colliculus (IC) of Mexican free-tailed bats. We presented 10 calls to each neuron of which 8 were social communication and 2 were echolocation calls. We also measured excitatory response regions: the range of tone burst frequencies that evoked discharges at a fixed intensity. The calls evoked highly selective responses in that IC neurons responded to some calls but not others even though those calls swept through their excitatory response regions. By convolving activity in the response regions with the spectrogram of each call, we evaluated whether responses to tone bursts could predict discharge patterns evoked by species-specific calls. The convolutions often predicted responses to calls that evoked no responses and thus were inaccurate. Blocking inhibition at the IC reduced or eliminated selectivity and greatly improved the predictive accuracy of the convolutions. By comparing the responses evoked by two calls with similar spectra, we show that each call evoked a unique spatiotemporal pattern of activity distributed across and within isofrequency contours and that the disparity in the population response was greatly reduced by blocking inhibition. Thus the inhibition evoked by each call can shape a unique pattern of activity in the IC population and that pattern may be important for both the identification of a particular call and for discriminating it from other calls and other signals.

Background noise improves gap detection in tonically inhibited inferior colliculus neurons

Single units in the inferior colliculus (IC) in the C57Bl/6 inbred mouse strain were tested for their temporal processing ability as measured by their minimum gap threshold (MGT), the shortest silent interval in an ongoing white-noise stimulus which a unit could encode. After ascertaining the MGT in quiet, units were re-tested in various levels of background noise. The focus of this report is on two types of tonically responding units found in the IC. Tonically inhibited (TI) units encoded gaps poorly in quiet and low levels of background noise as compared with tonically excited (TE) units. In quiet, the MGTs of TI units were about an order of magnitude longer than the MGTs typical of TE units. Paradoxically, gap encoding was improved in high levels of background noise for TI units. This result is unexpected from the traditional viewpoint that noise necessarily degrades signal processing and is inconsistent with psychophysical observations of diminished speech and gap detection processing in noisy environments. We believe the improved feature detection described here is produced by the adaptation of inhibitory input. Continuous background noise would diminish the inhibitory efficacy of the gap stimulus by increasing the latency to the onset of inhibition and decreasing its duration. This would allow more spontaneous activity to "bleed through" the silent gap, thus signaling its presence. Improved feature detection in background noise resulting from inhibitory adaptation would seem an efficient neural mechanism and one that might be generally useful in other signal detection tasks.

Reversible inactivation of the dorsal nucleus of the lateral lemniscus reveals its role in the processing of multiple sound sources in the inferior colliculus of bats

Neurons in the inferior colliculus (IC) that are excited by one ear and inhibited by the other [excitatory-inhibitory (EI) neurons] can code interaural intensity disparities (IIDs), the cues animals use to localize high frequencies. Although EI properties are first formed in a lower nucleus and imposed on some IC cells via an excitatory projection, many other EI neurons are formed de novo in the IC. By reversibly inactivating the dorsal nucleus of the lateral lemniscus (DNLL) in Mexican free-tailed bats with kynurenic acid, we show that the EI properties of many IC cells are formed de novo via an inhibitory projection from the DNLL on the opposite side. We also show that signals excitatory to the IC evoke an inhibition in the opposite DNLL that persists for tens of milliseconds after the signal has ended. During that period, strongly suppressed EI cells in the IC are deprived of inhibition from the DNLL and respond to binaural signals as weakly inhibited or monaural cells. By relieving inhibition at the IC, we show that an initial binaural signal essentially reconfigures the circuit and thereby allows IC cells to respond to trailing binaural signals that were inhibitory when presented alone. Thus, DNLL innervation creates a property in the IC that is not possessed by lower neurons or by collicular EI neurons that are not innervated by the DNLL. That property is a change in responsiveness to binaural signals, a change dependent on the reception of an earlier sound. These features suggest that the circuitry linking the DNLL with the opposite central nucleus of the IC is important for the processing of IIDs that change over time, such as the IIDs generated by moving stimuli or by multiple sound sources that emanate from different regions of space.

Latency as a function of intensity in auditory neurons: influences of central processing

The response latencies of sensory neurons typically shorten with increases in stimulus intensity. In the central auditory system this phenomenon should have a significant impact on a number of auditory functions that depend critically on an integration of precisely timed neural inputs. Evidence from previous studies suggests that the auditory system not only copes with the potential problems associated with intensity-dependent latency change, but that it also modifies latency change to shape the response properties of many cells for specific functions. This observation suggests that intensity-dependent latency change may undergo functional transformations along the auditory neuraxis. The goal of our study was to explore these transformations by making a direct, quantitative comparison of intensity-dependent latency change among a number of auditory centers from the lower brainstem to the thalamus. We found two main ways in which intensity-dependent latency change transformed along the neuraxis: (1) the range of latency change increased substantially and (2) one particular type of latency change, which has been suggested to be associated with sensitivity to temporally segregated stimulus components, occurred only at the highest centers tested, the midbrain and thalamus. Additional testing in the midbrain (inferior colliculus) indicated that inhibitory inputs are involved in shaping latency change. Our findings demonstrate that the central auditory system modifies intensity-dependent latency changes. We suggest that these changes may be functionally incorporated, actively enhanced, or modified to suit specific functions of the auditory system.

Duration tuning in the mouse auditory midbrain

Temporal cues, including sound duration, are important for sound identification. Neurons tuned to the duration of pure tones were first discovered in the auditory system of frogs and bats and were discussed as specific adaptations in these animals. More recently duration sensitivity has also been described in the chinchilla midbrain and the cat auditory cortex, indicating that it might be a more general phenomenon than previously thought. However, it is unclear whether duration tuning in mammals is robust in face of changes of stimulus parameters other than duration. Using extracellular single-cell recordings in the mouse inferior colliculus, we found 55% of cells to be sensitive to stimulus duration showing long-pass, short-pass, or band-pass filter characteristics. For most neurons, a change in some other stimulus parameter, (e.g., intensity, frequency, binaural conditions, or using noise instead of pure tones) altered and sometimes abolished duration-tuning characteristics. Thus in many neurons duration tuning is interdependent with other stimulus parameters and, hence, might be context dependent. A small number of inferior colliculus neurons, in particular band-pass neurons, exhibited stable filter characteristics and could therefore be referred to as "duration selective." These findings support the idea that duration tuning is a general phenomenon in the mammalian auditory system.

Neural measurement of sound duration: control by excitatory-inhibitory interactions in the inferior colliculus

In the inferior colliculus (IC) of the big brown bat, a subpopulation of cells ( approximately 35%) are tuned to a narrow range of sound durations. Band-pass tuning for sound duration has not been seen at lower levels of the auditory pathway. Previous work suggests that it arises at the IC through the interaction of sound-evoked, temporally offset, excitatory and inhibitory inputs. To test this hypothesis, we recorded from duration-tuned neurons in the IC and examined duration tuning before and after iontophoretic infusion of antagonists to gamma-aminobutyric acid-A (GABA(A)) (bicuculline) or glycine (strychnine). The criterion for duration tuning was that the neuron's spike count as a function of duration had a peak value at one duration or a range of durations that was >/=2 times the lowest nonzero value at longer durations. Out of 21 units tested with bicuculline, duration tuning was eliminated in 15, broadened in two, and unaltered in four. Out of 10 units tested with strychnine, duration tuning was eliminated in four, broadened in one, and unaltered in five. For units tested with both bicuculline and strychnine, bicuculline had a greater effect on reducing or abolishing duration tuning than did strychnine. Bicuculline and strychnine both produced changes in discharge pattern. There was nearly always a shift from an offset response to an onset response, indicating that in the predrug condition, inhibition arrived simultaneously with excitation or preceded it. There was often an increase in the length of the spike train, indicating that in the predrug condition, inhibition also coincided with later parts of excitation. These findings support two hypotheses. First, duration tuning is created in the IC. Second, although the construction of duration tuning varies in some details among IC neurons, it follows three rules: 1) an excitatory and an inhibitory event are temporally linked to the onset of sound but temporally offset from one another; 2) the duration of some inhibitory event must be linked to the duration of the sound; 3) an excitatory event must be linked to the offset of sound.

Temporal processing in sensory systems

The idea that sensory information is represented by the temporal firing patterns of neurons or entire networks, rather than by firing rates measured over long integration times, has recently gained increasing experimental support. A number of mechanisms that help to preserve temporal information in ascending sensory systems have been identified, and the role of inhibition in these processes has been characterized. Furthermore, it has become obvious that temporal processing and the representation of sensory events by temporal spike patterns are highly dependent upon the behavioral state of the animal or experimental subject.